This invention relates to a method for forming a coating composed of a polyimide layer having an electroless metal plate bonded thereto. More particularly, this invention relates to such method utilizing dispersed palladium nuclei deposited from a colloidal suspension to catalyze electroless deposition of the metal plate, which method includes a multi-step cure of the polyimide carried out at several stages of the electroless deposition process to improve seeding of the catalyst and enhance adhesion of the plate.
A typical substrate for a microelectronic component package comprises a copper plate applied to a polymeric dielectric layer and patterned to define the desired electrical leads. A convenient process for applying the copper plate is by electroless deposition. The polymer surface is sensitized by applying palladium-tin colloidal particles from an aqueous suspension. The particles are activated by treatment with fluoboric acid solution to dissociate the tin and produce catalytic palladium nuclei. The surface, seeded with the palladium nuclei, is treated with an aqueous alkaline solution containing a reducible cupric species and a reducing agent, typically formaldehyde. In the presence of the catalytic nuclei, the copper is reduced to deposit metallic copper onto the surface. The process is autocatalytic, so that the nascent copper metal catalyzes further deposition to produce a continuous plate. After an intitial flash of the copper is deposited electrolessly, additional copper may be electroplated onto the flash to form a product plate of the desired thickness.
Electroless copper deposition has been successfully developed for dielectric substrates formed of epoxy resin and the like. It is desired to produce a substrate formed of polyimide resin to take advantage of its low dielectric constant, high thermal stability and reduced moisture absorptivity. However, electroless copper plates deposited onto a fully cured polyimide layer by conventional processes suffer from poor adhesion. Common techniques for enhancing adhesion of an electroless plate on polyimide include physically or chemically etching to roughen the surface. Mechanical techniques that abrade the surface tend to be difficult to control to obtain uniform treatment. The most common technique for chemically etching utilizes caustic solution to attack the polyimide molecules and thus degrades the quality of the resin at the surface. In any event, even with such surface treatments, it has been possible to apply only a very thin layer of copper to polyimide before the plate separates from the surface due to the poor adhesion. Therefore, there remains a need to improve adhesion of electroless copper plate on polyimide resin to thereby permit a thicker plate to be applied without separation.